Your basal metabolic rate (BMR) is the number of calories your body burns at complete rest just to stay alive — powering your heart, lungs, brain, and cell repair. Knowing how to calculate BMR is the first step in working out how many calories you need each day, whether your goal is losing fat, building muscle, or simply eating with more confidence. This guide explains the Mifflin-St Jeor equation (the formula most dietitians rely on today), walks through worked examples for men and women, and shows you how to turn your BMR into a real daily calorie target. If you would rather skip the math, the free CalcRange BMR Calculator does it for you in seconds.
What Is BMR?
Basal metabolic rate (BMR) is the amount of energy, measured in calories, that your body uses in 24 hours while completely at rest — awake, fasted, and in a comfortable temperature. It covers only essential functions such as breathing, circulation, temperature regulation, and cell production, not movement or digestion.
BMR is the single largest part of your daily energy use. For most people it accounts for roughly 60 to 70 percent of the calories burned each day. Everything else — walking, training, fidgeting, and even digesting food — is added on top of it.
You may also see the term RMR (resting metabolic rate). RMR is measured under slightly less strict conditions, so it runs a little higher than BMR. For everyday planning, the two are close enough that most online tools treat them the same way.
How Does a BMR Calculation Work?
True BMR can only be measured in a lab using indirect calorimetry, where technicians analyze the oxygen you breathe in and the carbon dioxide you breathe out. That is accurate but expensive and impractical for most people.
Instead, researchers have built prediction equations from thousands of lab measurements. These formulas estimate BMR from four inputs that strongly predict how much energy a body needs:
- Weight — more body mass requires more energy to maintain.
- Height — taller bodies have more surface area and tissue.
- Age — metabolism gradually slows as lean mass declines with age.
- Sex — men typically carry more muscle, which burns more calories at rest than fat does.
A good formula gets most healthy adults within about 10 percent of their lab-measured value. That is close enough to build a sensible calorie plan, as long as you treat the result as a starting estimate rather than a fixed truth.
BMR Formulas Explained
Three equations dominate BMR calculation. All use metric units: weight in kilograms (kg), height in centimetres (cm), and age in years.
Mifflin-St Jeor equation (recommended)
Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5
Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161
Published in 1990 in the American Journal of Clinical Nutrition, the Mifflin-St Jeor equation is the one the Academy of Nutrition and Dietetics recommends for healthy adults because it tracks modern lab measurements most closely.
Revised Harris-Benedict equation
Men: BMR = 88.362 + (13.397 × weight) + (4.799 × height) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × weight) + (3.098 × height) − (4.330 × age)
The original Harris-Benedict formula dates from 1919 and was revised in 1984. It is still widely used, but it tends to estimate slightly higher than Mifflin-St Jeor for many people.
Katch-McArdle formula
BMR = 370 + (21.6 × lean body mass in kg)
This version ignores sex and age and works directly from lean body mass — your weight minus body fat. It is a strong choice if you already know your body fat percentage, which you can estimate with the body fat calculator.
How to Calculate BMR Step by Step
Here is the full process using Mifflin-St Jeor:
- Convert your measurements to metric. Divide pounds by 2.205 to get kilograms. Multiply inches by 2.54 to get centimetres.
- Multiply your weight in kg by 10.
- Multiply your height in cm by 6.25 and add it to the result.
- Multiply your age by 5 and subtract it.
- Apply the sex constant. Add 5 if you are male; subtract 161 if you are female.
- Read the result. The number is your estimated resting calorie burn per 24 hours.
If you prefer working in pounds and inches, an easier route is to enter your numbers directly into the online BMR tool, which handles unit conversion automatically.
Worked Examples
Example 1: 30-year-old woman, 65 kg, 165 cm
- 10 × 65 = 650
- 6.25 × 165 = 1,031.25
- 5 × 30 = 150
- BMR = 650 + 1,031.25 − 150 − 161 = 1,370 calories per day
Example 2: 40-year-old man, 80 kg, 178 cm
- 10 × 80 = 800
- 6.25 × 178 = 1,112.5
- 5 × 40 = 200
- BMR = 800 + 1,112.5 − 200 + 5 = 1,718 calories per day
In plain English: even if this man stayed in bed all day, his body would still burn roughly 1,700 calories keeping his organs running. Every step he takes adds to that number.
Calculate Your BMR Instantly
Enter your age, sex, height, and weight into the CalcRange BMR Calculator to get your result in seconds — no unit conversion or arithmetic required. It uses the same Mifflin-St Jeor equation explained above.
BMR vs TDEE: What Your Number Actually Means
Your BMR is not the number of calories you should eat. It is the floor — the energy cost of simply existing. To find your real daily need, called TDEE (total daily energy expenditure), multiply your BMR by an activity factor:
| Activity level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little exercise | 1.2 |
| Lightly active | Light exercise 1–3 days/week | 1.375 |
| Moderately active | Moderate exercise 3–5 days/week | 1.55 |
| Very active | Hard exercise 6–7 days/week | 1.725 |
| Extra active | Physical job plus hard training | 1.9 |
Using Example 1: 1,370 × 1.55 ≈ 2,124 calories to maintain weight for a moderately active woman. To lose weight gradually, she would eat somewhat below that TDEE — not below her BMR. The calorie calculator combines both steps and suggests targets for weight loss, maintenance, and gain, and the protein calculator helps you split those calories sensibly.
Factors That Affect Your BMR
- Muscle mass. Muscle tissue burns more calories at rest than fat tissue, so strength training raises BMR over time.
- Age. BMR declines gradually through adulthood, largely because lean mass declines. Staying active slows this drop.
- Hormones. Thyroid conditions can raise or lower metabolic rate significantly. If your energy levels or weight change unexpectedly, see a doctor.
- Genetics. Two people with identical stats can differ by a few hundred calories per day.
- Dieting history. Prolonged, aggressive calorie restriction can temporarily lower BMR as the body adapts (often called adaptive thermogenesis).
- Body size. Bigger bodies need more energy — which is why BMR falls as you lose weight and calorie targets need updating.
Common BMR Calculation Mistakes
- Eating at your BMR to lose weight. Your deficit should come off your TDEE. Eating at or below BMR for long periods is unnecessarily aggressive for most people.
- Mixing up units. Entering pounds where kilograms belong inflates the result badly. 150 lb is 68 kg, not 150 kg.
- Choosing an inflated activity level. Most people with desk jobs who train three times a week are “lightly” to “moderately” active, not “very active.”
- Treating the number as exact. Formulas carry a margin of error of roughly 10 percent. Track your real-world results for two to three weeks and adjust.
- Never recalculating. After every 5 kg (about 10 lb) of weight change, your BMR has meaningfully shifted — run the numbers again.
Frequently Asked Questions
What is a normal BMR?
Most adult women fall roughly between 1,200 and 1,500 calories per day, and most adult men between 1,500 and 1,900, but the range is wide. Body size, muscle mass, and age all shift the number, so compare your result with your own history rather than with other people.
Is BMR the number of calories I should eat?
No. BMR is your resting burn only. Multiply it by an activity factor to get your TDEE, and set your intake relative to that. Eating at BMR is effectively a steep deficit for anyone who moves at all during the day.
Which BMR formula is the most accurate?
For most healthy adults, Mifflin-St Jeor is the best-validated choice and the one professional dietetic bodies recommend. If you know your body fat percentage, Katch-McArdle can be more accurate for very muscular or very lean people.
Does losing weight lower my BMR?
Yes. A lighter body needs less energy, and a portion of the drop comes from metabolic adaptation. This is normal and expected — it simply means you should recalculate your targets as your weight changes.
Can I increase my BMR?
The most reliable long-term lever is building muscle through resistance training. Adequate protein, enough sleep, and avoiding extreme prolonged dieting also help keep your metabolic rate healthy. Supplements marketed as “metabolism boosters” have little meaningful effect.
Why do different calculators give me different numbers?
They likely use different equations — Harris-Benedict typically reads slightly higher than Mifflin-St Jeor for the same inputs. Differences of 50 to 150 calories between tools are normal and matter less than tracking your real results.
How often should I recalculate my BMR?
Recalculate after roughly every 5 kg (10 lb) of weight change, after a big change in training habits, or once a year as you age. It takes seconds with an online calculator, so there is no cost to checking.
Conclusion
Learning how to calculate BMR gives you the foundation every calorie decision rests on: multiply weight, height, and age through the Mifflin-St Jeor equation, apply the sex constant, then scale by your activity level to find your true daily need. Treat the output as a well-informed starting point, watch how your body responds for a few weeks, and adjust. When you want a quick answer — or want to re-run the numbers after progress — the free BMR calculator on CalcRange is the fastest way to get it, and pairs naturally with the BMI calculator for a fuller picture of where you stand.
Sources and Further Reading
- Mifflin MD, St Jeor ST, et al. “A new predictive equation for resting energy expenditure in healthy individuals.” American Journal of Clinical Nutrition, 1990.
- Roza AM, Shizgal HM. “The Harris Benedict equation reevaluated.” American Journal of Clinical Nutrition, 1984.
- U.S. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) — Body Weight Planner.
Further reading
For authoritative background on this topic, see Basal metabolic rate on Wikipedia.
Health disclaimer: This article and calculator provide general educational estimates and are not a substitute for professional medical advice, diagnosis, or treatment. Consult a qualified healthcare professional before making significant changes to your diet or exercise routine.
Last reviewed: July 2026. Recommended editorial review: every 12 months.